Okay kiddo, remember when we talked about atoms and how they can share electrons to make molecules? Well, sometimes when atoms share electrons, the electrons don't spend equal time with each atom.
Mesomeric, which is also called resonance, happens when atoms in a molecule share electrons in a special way that makes the electrons hang out with both atoms at the same time. It's kind of like having two best friends and spending time with both of them at the same time.
When this happens, it can change the way a molecule looks or behaves because the electrons are shared in a different way than usual. Scientists use special symbols to show when a molecule has mesomeric. It's like drawing a funny picture of two best friends hanging out together.
So, mesomeric is just a special way that some molecules can share electrons between atoms, and it changes the way the molecule behaves or looks. Cool, huh?